A hydrostatic hybrid drive system for road vehicles includes a pump/motor unit (1) connected to the powertrain of the vehicle, controlled by a control unit (5) in a pump or motor operation, and connected to a high-pressure hydraulic accumulator via a first working line (7). The first working line can be closed by a control valve (45) and connected to a low-pressure hydraulic accumulator via a second working line (9). The high-pressure hydraulic accumulator and low-pressure hydraulic accumulator are formed by a double-piston accumulator (11), in which a high-pressure side and a low-pressure side having respective accumulator pistons (15, 17) are formed in an accumulator housing (13). The fluid chambers (23, 25) of the high-pressure side and the low-pressure side are separated by a central housing part (21) through which the common piston rod for both accumulator pistons (15, 17) extends. The control unit (5) of the pump/motor unit (1) having a control area can be supplied from the first working line (7) via a line connection (59) provided on the first working line (7) between the pump/motor unit (1) and the control valve (45).